ss5_ble_generate_secret.py

from cryptography.hazmat.primitives import cmac
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives.ciphers import algorithms

# Sesamiの公開鍵
public_bytes = payload[3:67]

# private_bytesはアプリ側の秘密鍵
private_key_num = int.from_bytes(private_bytes, byteorder='big')

curve = ec.SECP256R1()
private_key = ec.derive_private_key(private_key_num, curve)

if len(public_bytes) == 64:
    public_bytes = b'\x04' + public_bytes

public_key = ec.EllipticCurvePublicKey.from_encoded_point(curve, public_bytes)
secret_key = private_key.exchange(ec.ECDH(), public_key)

# 永続化を推奨
secret = secret_key[0:16]

c = cmac.CMAC(algorithms.AES(secret))

# 初期化で受け取った４バイトの値
c.update(random_code)

# トークンは永続化しない
token = c.finalize()